As a method replacing a conventional radiography, a radiation image recording arid reproducing method utilizing a stimulable phosphor as described, for instance, in U.S. Pat. No. 4,239,968, was proposed and is practically employed. In the method, a radiation image storage panel comprising a stimulable phosphor (i.e., stimulable phosphor sheet) is employed, and the method involves the steps of causing the stimulable phosphor of the panel to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as "stimulating rays") to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals.
In the radiation image recording and reproducing method, a radiation image is obtainable with a sufficient amount of information by applying a radiation to an object at a considerably smaller dose, as compared with the conventional radiography using a combination of a radiographic film and radiographic intensifying screen. Further, the radiation image recording and reproducing method using a stimulable phosphor is of great value especially when the method is employed for medical diagnosis.
The radiation image storage panel employed in the above-described method has a basic structure comprising a support and a stimulable phosphor layer provided on one surface of the support. If the phosphor layer is self-supporting, however, the support may be omitted. Further, a transparent protective film of a polymer material is generally provided on the free surface (surface not facing the support) of the phosphor layer to keep the phosphor layer from chemical deterioration or physical shock.
The phosphor layer generally comprises a binder and a stimulable phosphor (in the form of particles) dispersed therein. The stimulable phosphor emits light (gives stimulated emission) when it is exposed to radiation such as X-rays and then excited with an electromagnetic wave (i.e., stimulating rays). Accordingly, the radiation having passed through an object or radiated from an object is absorbed by the stimulable phosphor layer of the panel in proportion to the applied radiation dose, and a radiation image of the object is produced on the panel in the form of a radiation energy-stored latent image. The radiation energy-stored image can be released as stimulated emission by sequentially irradiating the panel with the stimulating rays. The stimulated emission is then photoelectrically detected to give electric signals, so as to reproduce a visible image from the electric signals.
As described hereinbefore, the radiation image recording and reproducing method is a very advantageous radiation image reproducing method. Even in this method, it is desirable to give a reproduced radiation image of an improved quality such as high sharpness and good graininess with high sensitivity. The sensitivity of the radiation image storage panel essentially depends on the total amount of stimulated emission which is produced by the stimulable phosphor contained in the panel. The total amount of stimulated emission depends on the luminance of each stimulable phosphor particle as well as on the total amount of the stimulable phosphor particles contained in the phosphor layer of the panel. If the amount of stimulable phosphor in the phosphor layer is large, a large amount of radiation such as X-rays is absorbed by the phosphor layer, and hence a radiation image is obtained with a high sensitivity and a high graininess. It is also preferred to prepare the phosphor layer as thin as possible. This is because the thin phosphor layer reduces spread of stimulating rays in the phosphor layer by scattering and gives a reproduced radiation image of high sharpness.
U.S. Pat. No. 4,910,407 discloses a compression treatment of a stimulable phosphor layer formed on a support. Thus compressed stimulable phosphor layer contains phosphor particles therein at a density higher than the uncompressed stimulable phosphor layer and shows increased sharpness. However, the compressed stimulable phosphor layer sometimes shows decreased sensitivity and graininess as compared with the uncompressed stimulable phosphor layer, probably because a portion of the phosphor particles are broken down by the compression treatment.
U.S. Pat. Nos. 5,153,078 and 5,164,224 disclose a compression treatment of a stimulable phosphor layer comprising stimulable phosphor particles and a binder of a thermoplastic elastomer having a softening or melting point of 30.degree.-150.degree. C. at a temperature higher than the softening or melting point. Thus compressed stimulable phosphor layer shows increased sharpness with no decrease of graininess.
In the radiation image recording and reproducing method, the radiation image storage panel is repeatedly employed in the steps of radiation of X-rays (recording of radiation image), irradiation of stimulating rays (reading out of the recorded radiation image), and exposure to erasing light (erasure of residual radiation image). Between these steps, the storage panel is transferred by conveyors such as belts and/or rollers within an apparatus for the radiation image recording and reproducing method. It has been now found that the radiation image storage panel having the compressed stimulable phosphor layer is apt to produce therein cracks in its repeated use in the radiation image recording and reproducing system. The production of cracks are still observed even in the compressed stimulable phosphor layer using as thermoplastic elastomer having a softening or melting point of 30.degree.-150.degree. C. as the binder. The radiation image storage panel having a cracked stimulable phosphor layer cannot reproduce a radiation image of high quality because X-rays or stimulating rays impinged on the cracked phosphor layer is scattered on the cracked portion.